Project Summary/Abstract The Temporomandibular joint (TMJ) is considered to be the most complex and mysterious joint in the human body. There is a disc between the temporal bone and TMJ condyle whose cell origin is unclear. Its displacement leads to a specific TMJ disorder which causes TMJ pain and limited mouth opening. Second, there is an extra fibrous layer on the surface of condylar articular cartilage followed by a prechondroblastic layer. The cell origin and function for both are still unknown. Even more compelling, the mandible and TMJ condyle are commonly injured. Up to 75% of the injured TMJ condyle is able to regenerate itself with no need for surgery in children, while some patients developed into unilateral condylar hyperplasia for unknown reasons. Ironically, this joint has received the least attention compared to other joints or the growth plate. The TMJ condyle has abundant attachment sites of tendon, whose sole function has been thought to be transmitting muscle forces to stabilize joints. Tendon is also known to undergo ectopic ossification in trauma or diseases, although it is largely unclear if it has any direct contribution to normal skeleton formation. Considering all the information thus far, a series of in vivo experiments have been performed to search for a potential role of tendon in TMJ condyle formation and remodeling. The key findings are: 1) Scx+ (a tendon specific gene) cells in the fibrous layer directly contribute to TMJ condyle articular cartilage growth by transdifferentiating into prechondroblasts and chondrocytes; 2) Scx+ cells in the area under the tendon attachment play a key role in condyle expansion from a carrot-like to a broccoli- like shape by forming a fibrocartilage with a mixture of cartilage, bone and fibrous tissue 3) Scx+ cells directly transdifferentiate into chondrocytes followed by bone cells during TMJ condylar remodeling and regeneration; and 4) Scx+ cells are the major cell source for TMJ disc formation. Based on these findings, the central hypothesis is that tendon cells, beyond their role in joint movement, function as key precursors for TMJ disc formation, condyle growth, and joint remodeling. To test this central hypothesis, two highly related, yet independent Specific Aims are proposed: 1) To define the overlooked role of tendon in TMJ formation and growth beyond moving joints; and 2) To determine the vital role of tendon in TMJ remodeling via the regulation of mechanical force. Upon completion of the proposed work, it is expected to demonstrate that 1) tendon cells directly contribute to TMJ condyle chondrogenesis by forming fibrous-, prechondroblastic-, and chondrocyte-layers; 2) tendon cells are vital sources for TMJ condyle expansion by forming fibrocartilage; 3) tendon cells play critical roles in TMJ remodeling, which is mechanical force-dependent; and 4) tendon cells have a sole (or dominant) role of in TMJ disc formation and remodeling. Completion of this project is expected to have important impact because it is likely to fill the knowledge gaps in this largely unknown but vital area, revise the current dogma by raising a new type of tendon-derived chondrogenesis mechanism that is distinct from perichondrium-derived chondrogenesis, and lay the foundation for developing novel approaches for the restoration of TMJ in trauma and diseases.